1. Field of the Invention
This invention relates to a laser diode (semiconductor laser) pumped solid state laser in which a solid laser medium is pumped by a laser diode, and more particularly to a laser diode pumped solid state laser in which the pumping laser diode is driven by a drive current superposed with a high-frequency.
2. Description of the Related Art
As disclosed, for instance, in Japanese Unexamined Patent Publication No. 62(1987)-189783, there has been known a laser diode pumped solid state laser in which a solid laser medium is pumped by a laser diode.
In such a laser diode pumped solid state laser, noise is sometimes generated in light output of the solid state laser when the laser diode is subjected to mode hop due to change in temperature and/or drive current and light output of the laser diode fluctuates. That is, when the light output of the laser diode fluctuates with time as shown in FIG. 2A, the light output of the solid state laser fluctuates with change in the light output of the laser diode and noise is generated in the light output of the solid state laser as shown in FIG. 2B.
As the method of preventing fluctuation in the light output of the laser diode (generation of noise), the following methods have been known.
(a) A method disclosed in Japanese Unexamined Patent Publication No. 4(1992)-76974 in which a high-frequency higher than the response frequency of the solid laser medium is superposed on the drive current for a laser diode of a gain optical waveguide type or an index optical waveguide type.
(b) A method disclosed in Japanese Unexamined Patent Publication No. 7(1995)-154014 in which an index optical waveguide type broad-area laser is employed as the pumping laser diode.
The basic structures of the gain optical waveguide type laser diode and the index optical waveguide type laser diode will be described with reference to FIGS. 16 and 17, hereinbelow. In FIGS. 16 and 17, the hatched ellipse represents a cross-section of the beam, and reference numerals 1, 2 and 3 respectively denote a metal electrode, a low refractive index layer such as of SiO2 and an active layer.
When a laser diode is sufficiently large in the distance d between the active layer 3 and the low refractive index layer 2 as compared with the longitudinal beam diameter which is generally not larger than 1 xcexcm as shown in FIG. 16, the laser diode is of a gain optical waveguide having no index structure with respective to a transverse beam.
To the contrast, when the distance d between the active layer 3 and the low refractive index layer 2 is smaller than the longitudinal beam diameter in a laser diode as shown in FIG. 17, the laser diode is of an index optical waveguide having an index structure with respective to a transverse beam.
In the method of (a), when a gain optical waveguide type laser diode is employed, the current to be applied to the laser diode must be held not higher than the threshold in order to lower coherency of the pumping light beam emitted from the laser diode. In order to keep unchanged the mean output of the laser diode under this condition, the peak power must be at least twice the mean output. Since the peak power is thus close to the maximum output when the laser diode is driven at a high output, the laser diode can excessively deteriorate or fail.
To the contrast, in the method of (b), noise generated by the laser diode itself is reduced as compared with when a gain optical waveguide broad-area laser is employed. However a certain amount of noise is still generated.
In view of the foregoing observations and description, the primary object of the present invention is to provide a laser diode pumped solid state laser in which generation of noise due to fluctuation of the light output of the laser diode can be sufficiently suppressed.
In accordance with the present invention, there is provided a laser diode pumped solid state laser comprising
a solid laser medium, and
an index optical waveguide type laser diode which emits a pumping light beam for pumping the laser medium,
wherein the improvement comprises
a high frequency superposing means which superposes on a drive current for the laser diode a high-frequency which is higher than a response frequency of the solid laser medium and is of an amplitude which makes a percentage modulation of the light output of the laser diode a value not smaller than 50% and smaller than 100%.
Preferably the high-frequency is of an amplitude which makes the percentage modulation of the light output of the laser diode a value not smaller than 50% and not larger than 80%.
Preferably the frequency of the high-frequency is not lower than 20 MHz.
It is preferred that the laser diode be a multiple transverse mode broad-area laser.
Further it is preferred that the laser diode has index steps produced by a ridge structure.
The percentage modulation of the light output of the laser diode is defined as
xe2x80x83Pp-p/2PDCxc3x97100(%)
wherein PDC represents the mean light output (DC component) and Pp-p represents the peak-to-peak width of the light output of the laser diode. For example, when the light output of the laser diode changes as shown in FIG. 3B as the laser diode drive current changes as shown in FIG. 3A by superposition of the high-frequency, the percentage modulation is 100% since Pp-p=2PDC. When the light output of the laser diode changes as shown in FIG. 4B as the laser diode drive current changes as shown in FIG. 4A by superposition of the high-frequency, the percentage modulation is 50% since Pp-p=PDC.
Relation between the drive current and the amount of noise when the laser diode was driven without superposition of a high-frequency was measured for a pair of index optical waveguide type laser diodes and a gain optical waveguide type laser diode. These laser diodes were all multiple transverse mode laser diodes which were about 50 xcexcm in emission width. Result of the measurement is shown in FIGS. 5A to 5C. FIGS. 5A and 5C are for the index optical waveguide type laser diodes and FIG. 5C is for the gain optical waveguide type laser diode.
As can be understood from FIGS. 5A to 5C, noise was generated over a wide range of the values of the drive current in the case of the gain optical waveguide type laser diode. To the contrast, in the case of the index optical waveguide laser diodes, noise was generated only in an extremely limited range of the values of the drive current. From this fact, it is conceivable that when an index optical waveguide type laser diode is driven by a drive current superposed with a high-frequency, a large noise reduction effect can be obtained even if the amplitude of the high-frequency is relatively small. This invention has been made on the basis of this recognition.
FIG. 6 is a graph showing the relation between the percentage modulation and the amount of noise (to be described later) when a pair of index optical waveguide type laser diodes (LD1 and LD2) were driven with superposition of a high-frequency of 100 MHz. As can be understood from FIG. 6, when the percentage modulation is set not smaller than 50%, the amount of noise can be suppressed to not larger than 0.5%, which is a measure of the amount of noise practically permissible.
FIG. 7 is a graph showing the relation between the ratio of the mean light output to the maximum light output and the service life when a 800 nm band index optical waveguide type laser diode having a ridge structure was driven with superposition of a high-frequency. The laser diode was 10 to 300 xcexcm in stripe width and the relation shown in FIG. 7 was obtained when the laser diode was driven with the light output taken as 5 to 15 mW/xcexcmxc3x97stripe width (xcexcm) (e.g., when the stripe width is 50 xcexcm, the light output is 250 to 750 mW).
As can be understood from FIG. 7, when the ratio of the mean light output to the maximum light output is not larger than 1.8, that is, the percentage modulation is not larger 80%, a service life not shorter than 5000 hours, which practically gives rise to no problem, can be ensured.
It is needless to say that the percentage modulation may be larger than 80% for applications where a short service life gives rise to no problem. However from the viewpoint of the service life, it is preferred that the percentage modulation be as close to 50% as possible taking into account a variation in elements and the like.
When the percentage modulation of the light output of the laser diode is set not larger than 80%, the maximum light output becomes lower for a given mean light output as compared with when the percentage modulation of the light output of the laser diode is set 100% as in the past, and accordingly the laser diode can be driven with its maximum light output held low.